Rinse the lentils and cook them in a saucepan of boiling salted water, about 30 min until tender but still somewhat firm. Drain, discard the liquid and set aside the lentils.

Cook the quinoa in the water then let it cool down 10 minutes.

Prepare the vegetables: Seed, core and dice the pepper; finely chop the shallot; coarsely chop the arugula. Place the vegetables in a large bowl. Add the lentils and quinoa to the bowl.

In a small bowl, whisk together the oil, lemon juice, mustard, salt, and pepper. Pour over the salad. Gently toss to combine. Coarsely crumble the feta cheese and add it on top of the salad. Sprinkle with the finely chopped mint then serve.